This invention relates to hydraulic control means for controlling the advance rams of a tunnelling knife shield.
Knife shields are used for forming tunnels, drifts, mine galleries, adits, trenches, and other elongate excavations. Typically, a knife shield has a plurality of knives (planks) positioned side-by-side, and supported on a common support frame. A double-acting hydraulic ram is positioned between each of the rams and the support frame. The knives are advanced, either singly or in groups, by extending their rams, the remaining, stationary knives forming (with the support frame) an abutment for such advance movement owing to their frictional contact with the surrounding earth of the tunnel walls. When all the knives have been advanced, the support frame is advanced, in a follow-up step, by simultaneously retracting all the rams, the frictional contact between the knives and the surrounding earth providing an abutment for this movement.
In order to prevent the support frame from assuming undesired inclined positions, or from tilting, during its follow-up movement (and to ensure that all the rams are provided with substantially the same amount of hydraulic fluid), it is known to provide such a knife shield with control means for synchronising the retraction of the rams. For this purpose, flow-regulation valves are fitted in the hydraulic lines leading to the ram working chambers. Unfortunately, the flow-regulation valves available for this purpose can be used only for working pressures up to about 300 bars. At pressures higher than about 300 bars, these flow-regulation valves are subjected to intolerably heavy wear, so that their use cannot be entertained. On the other hand, knife shields usually operate at considerably higher hydraulic working pressures (500 bars or more), so that sufficiently large forces are available for advancing the knives rapidly.
The aim of the invention is to provide hydraulic control means for a knife shield, which control means can effect synchronous control of the knife advance rams at operating pressures of 500 bars or more, and without the flow-regulation valves being subjected to these high working pressures.